Photomechanical printing surface



Patented May 30, 1939 i STATES FATEv T OFFEE South India, assignor ofone-half to Robert Laing Bruce Gall, Ootacamund, Nilgiris, South IndiaNo Drawing. No. 28,572.

7 Claims.

This invention concerns improvements relating to the production ofprinting surfaces, including blocks, plates and the like, and has forits principal object to provide a process for producing these surfaceswhich is simpler or more economical than the processes hitherto known,and which is less affected or is unaffected, by temperature, humidityand similar factors.

In the process according to the invention, a

photographic originating element having the image defined in a halide ofsilver, or silver and mercury, is applied in contact with a smooth metalsurface, preferably a polished copper plate, in the presence of adeposition agent with which the halide does not react and in which it isonly slightly soluble and in the presence of which the halide is reducedby the metal of the smooth surface. In this manner a metallic silver ormercury-silver deposit corresponding to the image is obtained upon thesaid surface which may then be employed or treated as hereinafterexplained, depending upon the nature of the actual printing to beperformed. By a deposition agent is to be understood one of the reagentshereinafter referred to as such or an obvious chemical equivalentthereof.

The nature of the originating element employed, for instance whetherpositive or negative, will usually depend upon circumstances and thenature of the actual printing. In general the originating element may besimilar to that which would have been employed hitherto for printing ofthe same nature. For example, for the purposes of half-tone printing,the originating element may be a positive prepared with the assistanceof the usual screen.

In cases wherea developed positive is employed to give the originatingelement, the metal in which the image is defined is first converted intothe chloride, bromide or iodide thereof by a bleaching agent, forexample copper chloride, capable of effecting the conversion. As thestrength of the bleaching solution does not affect the quality of theimage, a strong solution may be used to save time, 10-30 secs. onlybeing then required. After a brief rinse, the element may at once beplaced in contact with the metal surface.

In cases where the positive employed has been obtained by a knownreversal process, the usual final development step [is unnecessary asthe image is then already defined in chloride, bromide or iodide.

Among the deposition agents which may be used for the treatment of thephotographic originating element are hydrochloric acid and certainApplication June 26, 1935, Serial In Great Britain July 5, 1934chlorides such as sodium or potassium chloride. An example of how theprocess may be carried into effect with hydrochloric acid will now begiven. In this example, the originating element having the image definedin silver chloride, bromide or iodide is pressed in contact with a cleanhighly polished copper plate (advantageously chemically clean and freefrom grease) which has been first wetted with hydrochloric acid dilutedwith water in, say, the ratio of two to five. A pressure sufficient toensure true contact between the element and copper plate is all that isnecessary. The effect of this contact, which is maintained for about oneto three minutes, is to deposit silver upon the copper plate in apolished form and in precise correspondence with the image originallydefined in the silver halide. A stronger acid solution or a longercontact period may be employed to give a heavy brilliant deposit, butthese are liable to lead to loss of polish upon the bare copper surfaceof the plate, which is undesirable. Should the polish be so lost,however, it may be restored without damaging the silver deposit bypolishing the plate with a soft cloth. Also, in order to obtain aheavier deposit,

the originating element itself may be immersed for a short time, saythirty seconds, in hydrochloric acid of the strength referred to beforebeing placed in contact with the plate.

Among other deposition agents which may be employed for the treatment ofthe originating element are: soluble sulphites, such as, for example,sodium sulphite, preferably used in aqueous solutions of about 30%strength.

As already stated, a deposit comprising mercury may be employed. Thisdeposit is apparently mercury with an admixture of silver in a smallproportion. The originating element for this modification may have theimage defined in a halide of silver and mercury, for example silvermercury-chloride. Such an image may be obtained, for example, bybleaching a photographic negative or positive in a mercuric halidesolution as is done in the common intensifying process employingmercuric chloride. sulphites, as referred to above, are preferably usedas the deposition agent for producing the metal deposit.

For some purposes, the copper plate upon which the deposit is made maybe replaced without disadvantage by a zinc plate electrolytically coatedwith copper or by a brass plate. Moreover, similar results can beobtainedwith, for example, a nickel-coated plate, a plate with anamalgamated surface or a zinc plate.

Further treatment of the surface bearing the metal deposit in order toadapt it for the actual printing will now be described:

In accordance with one such treatment, a copper plate with the silverdeposit is dipped into a solution obtained by adding a 12.5% solution ofcopper sulphate, containing 5% of concentrated sulphuric acid, to a 40%solution of hydrochloric acid (40 parts of hydrochloric acid of specificgravity 1.16 to 100 parts of water by volume) in proportions of 16:1. Acompound, which is presumably cuprous chloride, is formed on the exposedcopper, whilst the silver is unaffected. The plate is dried and one oftwo alternative courses followed: According to one course, the entiresurface of the plate is covered with printing ink applied by a rollerand the plate is immersed in a liquid in which cuprous chloride issoluble, for instance, a strong solution of sodium thiosulphate. The inkthat covered the cuprous chloride becomes loosened and can be removedby, for example, passing a roller there over, While that on the silverimage remains adhering. Etching may then be performed in the usualmanner. According to the other course, the silver is amalgamated byrubbing over the surface of the plate with metallic mercury. The cuprouschloride is then dissolved with a strong solution of sodium thiosulphateand the plate inked up. The ink adheres to the copper surface but not tothe amalgamated silver image. Etching may follow.

According to an alternative treatment, the plate with the silver depositis subjected to a graining step with the object of destroying the polishupon the copper surface whilst leaving that upon the silver surface, sothat subsequently the copper surface can be wetted Whereas the silverwill take the ink employed. This step may comprise treating the platethe plate, which should be free from grease, with a reagent whichattacks copper, such as a dilute solution of ferric chloride. Forexample, a solution of from 1-2% strength will require about 6030seconds to produce the necessary graining effect. It is possible tocheck when the effect has proceeded far enough, for the silver depositthen appears black against a pinkish background. The graining should notbe carried too far or the silver deposit may be impaired. The plate isnext inked up in the known manner for lithographic printing. It is to berecommended that the inking up should be performed in a lithographicmachine and that the whole surface of the plate should be kept moistduring the operation.

If after the first inking up, it appears that the graining isinsufilcient, it can still be accentuated by applying a strongerreagent. For certain inks, the above described graining step and thefirst inking up are in any case advantageously followed by theapplication to the plate of mercury or a solution of a mercuric salt.This has the effect of amalgamating or coating the bare copper partswhilst leaving the silver, which is covered by ink, unaffected. Theplate is then ready for lithographic printing and in this case the usualwetting is unnecessary.

When the plate is required for half-tone or line printing, the etchingmay be performed thereon, after the inking, in the usual manner. If aline negative originating element is employed, an engraved plate can beobtained such as is commonly employed for printing bank notes.

If it is required to obtain a positive printing surface from a negativeoriginating element, the above described graining and inking up may befollowed by the deposition of chromium upon the bare copper. The removalof the ink then leaves a positive image in chromium surrounded by theoriginal deposit of silver. Finally an etching agent, such as chromicacid, is applied to remove the silver and etch the copper to the depthrequired for printing.

The above-described graining step may be replaced by a step of which theobject is to deposit upon the bare copper of the plate an amount ofmercury which is sufficient to repel printing ink but which does notmaterially affect the silver deposit. As a very thin film of mercury issufficient if the copper surface is highly polished and the mercurydeposits in a brilliant form, care should be taken to maintain orrestore the polish in the copper. The mercury may be deposited bydipping the plate or immersing it for a short time, for instanceseconds, in a dilute solution of mercuric chloride, for example anaqueous 0.25% solution containing some free hydrochloric acid (forexample 8% measured by volume, of concentrated hydrochloric acid-sp. gr.1.16). The latter component is believed to assist in producing amirror-brilliant deposit of mercury.

The plate is next dipped in water for a second or so and dried with anabsorbent material, care being taken not to rub the surface.

The surface, which at this stage consists of the silver image surroundedby mercury, is then inked rapidly with a printing roller charged with avery thin layer of ink, whereupon the silver attracts ink whilst themercury repels it. The amount of ink attracted to the silver in thisoperation may not be heavy enough to resist the action of an etchingbath. To enable the ink deposit to be strengthened, the mercury depositmust be augmented. This may be done by immersing the plate for a shorttime in a stronger solution of mercuric chloride containing some freehydrochloric acid. Alternatively it may be done more rapidly by passingover the plate a copper or brass cylinder of which the surface haspreviously been amalgamated. This augments the existing mercury depositbut leaves the lightly inked parts unaffected. In either case, inking upmay then be performed with a roller charged with a normal amount ofprinting ink. The inking operations may be performed by hand or,preferably, by machine. etched and printed from in the ordinary manner.

By a modified process, it is possible to obtain a printing surface whichis of the opposite nature to the originating element, for example apositive printing surface directly from a negative originating elementor vice versa. Consequently, by adopting the appropriate process, it ispossible to produce a required printing surface from any given orconvenient originating element. Briefly the modified process resides intreating the plate in such manner as to cause mercury to adhere to thesilver deposit but so as ultimately to leave the copper unaffected. Inthis condition, the amalgamated silver surface will repel ink whereasthe copper surface will take it.

A specific example of the production of a positive printing surface froma negative originating element by this modification will now bedescribed: A silver deposit upon a copper plate is produced from theoriginating element as previously explained. The whole surface of theplate is amalgamated either directly with metallic mercury or byimmersion in a dilute solution of mercuric chloride containing some freehydro- Ihe plate is finally t chloric acid, the precise strength of suchsolution being of no great importance. In either case the surface isgently rubbed, for example with a cotton-wool pad, to facilitatethorough amalgamation of the silver. The amalgamation may require from30-60 seconds. After treatment under these conditions,the mercury doesnot adhere as firmly to the copper surface as it does to the silverdeposit and this fact is utilised in order to remove the mercury fromsaid copper surface. To this end, the whole surface of the plate isgiven an intimate coating of an adhesive medium, preferably printingink, which can be applied by rolling slowly and with pressure. Thesurface is then treated with a solvent as for example petrol or benzene,to remove or wash off the adhering ink or other adhesive medium. Theless strongly adhering mercury on the copper is removed at the sametime, leaving the said copper bare, whereas the greater part of the morestrongly adhering mercury amalgamated with the silver remains. It is tobe assumed that this effect is due to the mercury particles adheringless strongly to the smooth copper surface, and more strongly to thesilver deposit, than to a medium such as printing ink. The plate mayfinally be inked up, the link being attracted to the copper and repelledby the amalgamated silver, and may then be etched.

To assist the etcher, wet ink resists produced in the above describedmethods of operation are preferably dried in known manner by dustingthem over with bitumen and heating.

According to a modification, the ink image obtained upon the originalcopper plate is not used directly for lithographic printing, but istransferred to an appropriately grained zinc or aluminium plate by theoff-set method and the latter plate used for the printing.

An intaglio plate for rotary photogravure printing may be obtained inthe following manner: A film is exposed under a normal rotaryphotogravure screen and a negative of the original picture is then madeupon it. This developed negative is bleached and used as the originatingelement for depositing the silver image on the copper plate. The plateis then treated with a somewhat stronger solution of mercuric chloride(1 2 containing some free hydrochloric acid (16%) by volume ofconcentrated hydrochloric acid, (sp. gr. 1.16). The mercuric chloridedissolves the copper, depositing on it at the same time an amount offree mercury equivalent to the weight of copper dissolved. This solventaction of the mercuric chloride on the copper is retarded, where thesilver image occurs, according to the thickness of the silver depositwhich in turn corresponds, from point to point, to the tone in theoriginating negative. This retarding effect of the silver deposit is dueto the fact that the underlying copper can only be dissolved after thesilver above it has been amalgamated. The action of the mercuricchloride is continued until the thickest parts of the silver depositbegin to amalgamate (4-10 min). The deposited mercury is then evaporatedby heating and the resulting plate is of an intaglio nature suitable forrotary photogravure printing,

Alternatively the plate with the silver positive deposit and a plaincopper plate, connected respectively to the positive and negativeterminals of a battery, are placed in a copper salt solution, preferablya copper sulphate solution containing a small quantity of sulphuricacid. Upon the passage of a small current between the plates, it isfound that the copper under the deposited silver is everywhere dissolvedto a depth varying with the thickness of the silver deposit from pointto point. This gives a plate of intaglio nature which may be utilisedfor rotary photogravure printing.

According to a further alternative, a copper plate'with a silver depositobtained from a positive originating element printed under theappropriate screen is immersed in a weak iodine solution and then in adilute solution of sodium thiosulphate, ammonium or potassium cyanide.The surface then consists of silver iodide, into which the silver hasbeen converted, surrounded by copper. The plate is next made the cathodeof an electrolytic cell whereof the anode is nickel and the electrolytea solution of a nickel salt. Upon the passage of a small current throughthe cell, a black deposit, which is presumably nickel, is formed on thesilver iodide in accordance with the thickness of the latter which inturn corresponds from point to point with the tones of the originatingelement. This deposition may require a few minutes. Finally the plate isdried and an electrotype or cast is made from it to provide an intaglioplate which may be printed from in the normal manner.

As will be readily understood, the printing surfaces obtained accordingto the invention may also be employed in known manner for variouspurposes indirectly associated with printing. For example, if thesurface ultimately obtained is in cylindrical form, it can be employedor adapted for use for the transmission of the image by telegraph or bywireless.

I claim:

1. Process for the production af a printing surface from a photographicoriginating element having the image defined in a halide of silver,comprising applying the said element in contact with a smooth metalsurface in the presence of a deposition agent with which the halide doesnot react and in which it is only slightly soluble and in the presenceof which the halide is reduced by the metal of the smooth surface, andthereby producing upon the said surface a metallic silver depositcorresponding to the image.

2. Process for the production of a printing surface from a photographicoriginating element having the image defined in a halide of silver andmercury, comprising applying the said element in contact with a smoothmetal surface in the presence of a deposition agent with which thehalide does not react and in which it is only slightly soluble and inthe presence of which the halide is reduced by the metal of the smoothsurface, and thereby producing upon the said surface a metallicmercury-silver deposit corresponding to the image.

3. Process for the production of a printing surface from a photographicoriginating element having the image defined in a halide of silvercomprising applying the said element in contact with a polished copperplate in the presence of a partial solvent for the halide which solventis chemically inactive towards the said halide, to reduce the halide andproduce upon the said plate a metallic silver deposit corresponding tothe image.

4. Process for the production of a printing surface from a developedphotographic positive having the image defined in silver, comprisingtreating the said positive with a bleaching agent to convert the silverinto silver halide and then applying it in contact with a smooth metalsurface in the presence of a deposition agent with which the halide doesnot react and in which it is only slightly soluble and in the presenceof which the halide is reduced by the metal of the smooth surface, toproduce upon the said surface a metallic silver deposit corresponding tothe image in the positive.

5. Process for the production of a printing element from a photographicoriginating element having the image defined in a halide of silver,comprising applying the said originating element in contact with asmooth metal surface in the presence of a halide solution with which thesilver halide does not react and in which it is only slightly solubleand in the presence of which it is reduced by the metal of the smoothsurface,

and thereby producing upon the said surface a free metallic silverdeposit corresponding to the image.

6. Process for the production of a printing element from a photographicoriginating element having the image defined in a chloride of silver,comprising applying the said originating element in contact with asmooth copper surface in the presence of a chloride with which thesilver chloride does not react and in which it is only slightly solubleand in which it is capable of being reduced by the copper, and therebyproducing upon the said surface a metallic silver deposit correspondingto the image.

7. Process for the production of a printing element from a photographicoriginating element having the image defined in a halide of silver,comprising applying the said originating element in contact with asmooth metal surface in the presence of a solution of a chloride of analkali metal and thereby producing upon the said surface a metallicsilver deposit corresponding to the image.

KEVIN WILLIAM CATON W'EBB.

